How to do a 30-30-30 reset?

There is no 30/30/30 reset in Openwrt/LEDE.
That advice is generally from ancient times with dd-wrt etc. for routers that had “shortable” nvram memory modules (instead of modern flash, where all settings are normal files).

Source: https://forum.openwrt.org/t/linksys-e3000-reboot-loop/7771/2

OpenWrt provides three different methods for “starting over” with your router. See the Failsafe Mode, Factory Reset, and Recovery Mode article.